1. Technical Field
The embodiments herein generally relate to China Multimedia Mobile Broadcasting (CMMB), and, more particularly, to coarse carrier frequency offset estimation for CMMB mobile television (TV) receivers.
2. Description of the Related Art
CMMB is a mobile television and multimedia standard based on the satellite and terrestrial interactive multiservice infrastructure (STiMi). It specifies usage of the 2.6 GHz frequency band and occupies 25 MHz bandwidth within which it provides 25 video and 30 radio channels with additional data channels. Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation scheme, which uses a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is modulated with a conventional modulation scheme (e.g., quadrature amplitude modulation) at a low symbol rate, maintaining data rates similar to conventional single-carrier modulation schemes in the same bandwidth. Because of its multiple advantages (e.g., high spectrum efficiency, resistance against multi-path interference, and ease of filtering out noise etc.), OFDM has been widely used in wireless transmission technology.
However OFDM requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation, the sub-carriers are no longer orthogonal, causing inter-carrier interference (ICI) or cross-talk between the sub-carriers. Frequency offsets are typically caused by mismatched transmitter and receiver oscillators, or by Doppler shift due to movement. While Doppler shift alone may be compensated for by the receiver, the situation is worsened when combined with multipath factors, as reflections will appear at various frequency offsets, which is much harder to correct. This effect typically worsens as speed increases, and is an important factor limiting the use of OFDM in high-speed vehicles. Several techniques for ICI suppression are suggested, but they may increase the receiver complexity. CMMB uses the OFDM technique to achieve low cost equalizer implementation, large frequency diversity gain, and high spectrum efficiency.